Indeed, any message received by a mobile terminal is scrambled upon transmission and must therefore be descrambled by means of the same series of sequences as that initially generated by the transmitting base station. In addition, this descrambling must be performed in synchronism with the scrambling performed by the base station.
In the context of WCDMA (“Wideband Code-Division Multiple Access”), i.e. for one of the essential technologies necessary for implementation of the new generation (3G) of cellular systems, all of the data received is diffused and scrambled in the form of OVSF (“Orthogonal Variable Spreading Factor”) scrambling sequences and of scrambling sequences. Each OVSF sequence is periodic at the level of the symbols (or bit period) that compose it, just as each scrambling sequence is also periodic at the level of the time intervals (frame) that segment it.
To recover the message transmitted in the form of a signal at the receiving terminal, it is therefore necessary to recover the synchronism, between the sequences produced by the receiving terminal and the OVSF and scrambling sequences, transmitted by the base station. If this synchronism is not achieved, the signal received then simply amounts to noise.
All of the aforementioned synchronisation constraints are particularly present in UMTS-type mobile telecommunications terminals, and more specifically in the following two essential elements included in such a mobile terminal:                the RAKE receiver, which includes, inter alia, the technical component responsible for control of the synchronisation, and in particular that associated with the CPICH (“Common Pilot Indicator Channel”) pilot channel;        the cell detector, better known under the name “cell searcher”, which functions according to at least the following three main steps:                    step 1: search for the primary channel or “Primary Synchronisation Channel” (P-SCH);            step 2: synchronisation performed on the secondary channel, or “Secondary Synchronisation Channel” (S-SCH);            step 3: measurement of correlation on the common pilot indicator channel or CPICH.                        
We will now consider the hypothesis according to which the synchronisation time of the signal received is known, in other words the beginning of scrambling sequence is known. This information is available in particular from the “cell searcher” which delivers, in its operation steps one and two, the synchronisation sequences of the slot and frame time intervals for the signal received, completed by the multiple path detector (or “Multi-path searcher”), which delivers a phase synchronisation for each echo.
To demodulate the data of the signal received, it is thus necessary to initialise the scrambling code sequences and the OVSF sequences in a predetermined state, at a precise instant. When the scrambling sequence is known, the OVSF sequence to be used is provided by other channels of the protocol.
The problem presented, however, is that of knowing how to load the right value at the right time into the polynomial generating scrambling sequences.
However, as the operation of these two “Rake receiver” and “Cell searcher” elements consumes a relatively large amount of electrical energy during use of the terminal, they are alternatively turned on or off according to the receipt of messages, which requires finding the right synchronisation value for initialisation of the generator polynomial, at the right time, at each restart.
However, these successive shutdowns and restarts involve a notable convergence time for returning the generator polynomials “x” and “y” to the state that they occupied at the time of the shutdown, which confirms the benefit of providing an effective solution to the aforementioned problems.
1. Solutions of the Prior Art
Among the solutions of the prior art currently known, the most commonly used for attempting to optimise the convergence time to return the polynomials “x” and “y” to a predetermined specific state is the “slewing” technique. This technique is based on the fact that the receiver of a mobile telecommunications terminal comprises two periodic generators of bit sequences, identical to those of the base station. Thus, this terminal must first load the scrambling value into the generator of the in-phase channel, then recover the synchronism (“slewing”).
Two cases can then occur according to the value of the known time shift between the sequences that it produces and those transmitted by the base station:                if it is late, i.e. if the sequences at the output of its generators precede the corresponding sequences of the scrambled message in the series of sequences, the receiver will have to time the two generators at a frequency greater than the frequency of reception (equal to 3.84 Mchips/s) of the sequence in order to catch the correct phase;        if it is ahead, i.e. if the sequences at the output of its generators appear after the corresponding sequences of the scrambled message in the series of sequences, the receiver will have to freeze the two generators (no clock signal), until it has reached the correct phase.        
The “slewing” technique therefore amounts to accelerating or slowing (or even freezing) the generator polynomial so as to reach the desired state corresponding to an alignment of the data received.
Naturally, during the synchronisation delay necessary for the terminal to recover the synchronism, this terminal cannot descramble the message received, so it loses the information and expends energy unnecessarily. It is therefore important to minimise this synchronisation delay which is dependent on the processing time imposed by the “slewing” technique, which is generally on the order of a time interval of 2560 bits.
Very long periodic generators are generally used to produce sequences of pseudorandom bits. Such a generator is generally produced by means of a linear feedback shift register timed to the rhythm of a clock signal. The sequence bits generated correspond to the outputs of the flip-flops of the register.
A typical application of the periodic generator is scrambling. For clarification, reference can be made to the transmission mode used in the UMTS (“Universal Mobile Telecommunication System”) mobile communications system.
At the level of a base station, a message to be transmitted is modulated on two channels, the phase I channel and the quadrature Q channel. Each of the channels I and Q is scrambled by means of a system of two generators (“x” and “y”), although the state of the generator X is offset with respect to the state of Y by a scrambling value characterising the base station. This scrambling value (“scrambling code”) corresponds to a predetermined number of clock cycles. When the terminal is late, it is possible to adjust the frequency of the clock signal, which then makes it possible to perform clock jumps and thus to make up for the delay observed.
Consequently, each time the demodulator is on, it is caused to run idle until it converges, before beginning to demodulate. It then becomes necessary either to freeze or to accelerate the pseudorandom generators so as to compensate for their advancement or delay with respect to the frame of the signal received and thus enable them to be positioned at the state that they occupied at the time of the shutdown. A significant problem associated with this operating mode, however, relates to the fact that the convergence time is too high, which is generally close to the duration of a synchronisation time interval (frame). However, it currently appears to be impossible to dispense with such pseudorandom generators in order to initialise the registers at the right values and thus recover the synchronism between the bit sequences produced by the demodulator of the receiving terminal and the sequences transmitted by the base station.
A second solution known from the prior art for reducing this waiting time consists of skipping a predetermined number of sequences (generally called “immediate shift”) to the periodic generators. However, this so-called “mask storage” solution significantly increases the complexity of the generators and the production cost of such devices in a manner that is not necessarily justified for a UMTS-type mobile telephone terminal.
2. Disadvantages of the Prior Art
A first disadvantage of these prior art techniques is that they impose a synchronisation delay that is sometimes significant but necessary for the terminal to recover the synchronism, which terminal is incapable of directly descrambling the message received, resulting in a possible loss of information and unnecessary energy expenditures.
A second major disadvantage associated with these prior art techniques is that they significantly increase the complexity of the generators to be implemented, which usually use hardware solutions, which may be unsuitable given the miniaturisation constraints of mobile radiocommunication terminals and the components that they integrate.